1. Field of the Invention
The present invention relates to an exhaust purification system and method for an internal combustion engine, which is applied to an air-fuel ratio control system of an internal combustion engine for performing a lean combustion at a lean air-fuel ratio, and has a NOx occluding and reducing catalyst for purifying nitrogen oxides (NOx) in exhaust gases emitted at the time of the lean combustion.
2. Related Art
In an air-fuel ratio control system for an internal combustion engine in recent years, a technique for performing a so-called lean combustion control for burning fuel on the lean air-fuel ratio side relative to the stoichiometric ratio in order to improve the fuel consumption is being used more and more. In the case of carrying out the lean combustion, exhaust gases emitted from the internal combustion engine include a large amount of NOx, so that a NOx catalyst for purifying NOx is necessary. On the other hand, since fuel and lubricating oil contain sulfur, sulfur is contained in exhaust gases emitted from the internal combustion engine and adsorbed on the NOx catalyst as well as NOx. When sulfur is adsorbed on the NOx catalyst, the NOx adsorbing power is lessened. Consequently, techniques for removing sulfur adsorbed on the NOx catalyst is proposed.
For example, when platinum Pt and barium Ba are carried on a carrier, sulfur is adsorbed on the NOx catalyst, thereby forming stable sulfate BaSO.sub.4. When the amount of sulfate BaSO.sub.4 on the NOx catalyst increases, the NOx occlusion amount that the NOx catalyst can occlude is gradually purified.
According to JP-A No. 10-54274, when the sulfur adsorption amount exceeds a predetermined amount, the air-fuel ratio is controlled to the rich air-fuel ratio side and a heat generation of exhaust gases is increased. As a specific means, lean misfire is caused for a predetermined period and the air-fuel ratio is controlled to the rich side after the misfire. By this operation, unburned components are discharged to the NOx catalyst and burnt in the catalyst, thereby increasing the temperature of the catalyst. As a specific second means, the air-fuel ratio is controlled to the rich side and the ignition timing is retarded, thereby increasing the temperature of exhaust gases.
The exhaust purification system of the publication has, however, the following problems. Specifically, when the lean misfire is forcefully caused, unexpected torque fluctuations occur and the drivability is lessened with the torque fluctuations. Further, it is likely that unburned components such as HC and CO are discharged into the atmosphere in association with the misfire. In the case of retarding the ignition timing, it is necessary to increase the intake air volume in order to assure an output torque. The exhaust gas amount increases in association with it, so that it is likely that a total amount of harmful components such as HC, CO, and NOx increases.